Production of semiconductor devices requires formation of finer lithography patterns for increasing the degree of integration. A key technique for forming such micropatterns is use of a radiation (light) source of short wavelength (as used herein, “light” refers to a type of radiation). In addition to g-line and i-line, which have hitherto been used, deep-UV radiations such as krypton fluoride (KrF) excimer laser radiation (wavelength: 248 nm) and argon fluoride (ArF) excimer laser radiation (wavelength: 193 nm) have been introduced for mass production of semiconductor devices. Meanwhile, studies have been conducted on lithography techniques employing, as a radiation source, fluorine dimer (F2) excimer laser radiation (157 nm), extreme ultraviolet radiation (EUV), an electron beam (EB), or the like.
In the lithographic techniques, a resist film is employed for transferring a pattern to a substrate, and a variety of thin films may be optionally attached on or under the resist film. These thin films are formed through preparing a composition containing a polymer having a function of interest and an additive dissolved in an organic solvent, applying the composition to a substrate through spin-coating or a similar technique, and removing the solvent by optional heating or by curing the composition.
Resist films are divided into two types: a positive type resist; i.e., an irradiated portion is dissolved in a developer, and a negative type resist; i.e., a non-irradiated portion is dissolved in a developer. There have also been known, for example, a resist containing a compound whose solubility in a developer is varied by the effect of radiation, and a polymer as a binder which is soluble in an alkaline developer; a resist containing a compound which generates an acid by the effect of radiation (hereinafter may be referred to as “radiation-sensitive acid-generator”), and a polymer as a binder whose solubility in an alkaline developer is varied by the action of an acid. Particularly, the latter resist is called a chemical amplification-type resist, inter alia, a chemical amplification-type positive resist is particularly preferably employed in fine pattern processing.
As thin films formed on or under the resist film, there have been known, for example, an anti-reflection film, a gap-filling film, and a top coating film. The anti-reflection film is provided on a surface of a high-reflective substrate (forming a layer under the resist film) or a surface of the resist film (forming a layer on the resist film), to thereby suppress light reflection at the interface between the resist film and the reflective substrate and a standing wave, whereby fine resist patterns are accurately formed. The gap-filling film is provided on a surface of a substrate (forming a layer under the resist film or an anti-reflection film) in resist pattern formation on the pattern-formed substrate, whereby gaps present in the substrate surface are filled therewith, to thereby planarize the surface. The top coating film is provided on the resist film in immersion photolithography, in order to prevent migration of immersion liquid to the resist film and release of components such as a radiation-sensitive acid-generator from the resist film.
In the aforementioned composition in solution form for forming thin film, the polymer for lithography is an important component which is required to have optical, chemical, and physical properties so as to exhibit a function of interest to the thin film. Thus, extensive studies on the polymer for lithography are currently being carried out.
For example, as has been known, a chemical amplification-type positive resist with a KrF excimer laser employs, for example, a copolymer having a repeating unit derived from hydroxystyrene; and a repeating unit in which a phenolic hydroxyl group derived from hydroxystyrene is protected by an acid-dissociable dissolution-inhibitive group (e.g., an acetal structure or a tertiary hydrocarbon group), or a repeating unit in which a carboxyl group derived from (α-alkyl)acrylate is protected by an acid-dissociable dissolution-inhibitive group (e.g., an acetal structure or a tertiary hydrocarbon group) (see, for example, Patent Documents 1 to 4). There has also been known a copolymer having a repeating unit in which an alicyclic hydrocarbon group serves as an acid-dissociable dissolution-inhibitive group for increasing the dry etching resistance of the copolymer or for increasing the difference in rate of dissolution of the copolymer in an alkaline developer between before and after light exposure (see, for example, Patent Documents 5 and 6).
As candidates for a chemical amplification-type positive photoresist with ArF excimer laser radiation, there have been studied copolymers having no repeating unit which is derived from hydroxystyrene having a large absorption coefficient with respect to a wavelength of 193 nm. Actually, there have been known such copolymers formed of repeating units having a lactone structure, which is a polar group for enhancing adhesion to a semiconductor substrate or the like and regulating solubility of the copolymers in a solvent for lithography or in an alkaline developer (see, for example, Patent Documents 7 to 10).
As polymers for forming anti-reflection film, there have been known polymers formed of repeating units having an aromatic ring (e.g., benzene ring, naphthalene ring, or anthracene ring) serving as a functional group for enhancing absorption coefficients to wavelengths of 248 nm and 193 nm and refractive indexes of the polymers, and optional repeating units having a reactive functional group (e.g., amino, amide, hydroxyl, or epoxy), which reacts with a curing agent or the like to cause curing, for preventing intermixing with resist film (see, for example, Patent Documents 11 to 14).
As polymers for forming gap-filling film, there have been known polymers formed of repeating units having a reactive functional group, which has an appropriate viscosity suitable for pouring into narrow gaps and which reacts with a curing agent or the like to cause curing, for preventing intermixing with resist film or anti-reflection film. One specific example is a polymer formed of repeating units derived from hydroxystyrene and optional repeating units derived from a polymerizable monomer (e.g., styrene, alkyl (meth)acrylate, or hydroxyalkyl (meth)acrylate) (see, for example, Patent Document 15).
As copolymers for forming top coating film for immersion lithography, there have been known, for example, copolymers formed of repeating units having a carboxyl group (see, for example, Patent Document 16) and copolymers formed of repeating units having a hydroxyl group-substituted fluorine-containing group (see, for example, Patent Document 17).
In a trend for producing finer device patterns, there is keen demand for attaining uniformity in performance of a polymer from production lot to production lot (hereinafter “production lot” may be referred to as simply “lot”). In particular, when the weight average molecular weight (hereinafter may be referred to as “Mw”) of the polymer varies, variations arise in the thickness of the polymer film formed through lithography, exposure energy for producing the polymer film, and lithographic characteristics such as dissolution rate in alkaline developer, raising problems such as failure to form consistent lithographic patterns. Therefore, there is demand for a method for producing a polymer, which method can considerably reduce the difference in Mw between production lots of the polymer.    Patent Document 1: JP-A-1984-045439    Patent Document 2: JP-A-1993-113667    Patent Document 3: JP-A-1998-026828    Patent Document 4: JP-A-1987-115440    Patent Document 5: JP-A-1997-073173    Patent Document 6: JP-A-1998-161313    Patent Document 7: JP-A-1997-090637    Patent Document 8: JP-A-1998-207069    Patent Document 9: JP-A-2000-026446    Patent Document 10: JP-A-2001-242627    Patent Document 11: JP-A-2000-313779    Patent Document 12: JP-A-2001-27810    Patent Document 13: JP-A-2001-192411    Patent Document 14: JP-A-2001-226324    Patent Document 15: JP-A-2003-57828    Patent Document 16: JP-A-2006-193687    Patent Document 17: JP-A-2006-243308